Emulsion inks used in lithography are made from an emulsion of an oil-based ink solution and a water-based fountain solution. The emulsion is applied to a printing plate (typically mounted to a plate cylinder) having distinct image areas and non-image areas. The image areas have an oleophilic material, such as an oleophilic polymer, disposed on the surface thereof, so that the oil-based ink solution will adhere thereto for subsequent transfer to a printing substrate, such as a paper web. The non-image areas have a hydrophilic material, such as an aluminum oxide, disposed on the surface thereof, so that the water-based fountain solution will adhere thereto, thereby forming a protective film over the non-image areas, to prevent ink from adhering thereto. A principal advantage of the use of emulsion inks is that emulsion inks can eliminate the need for a separate system to dampen the printing plate and hence the use of emulsion inks eliminates printing problems associated with keeping the ink and water properly in balance. Also, using emulsion inks simplifies the printing process by eliminating the need for many ink keys that would otherwise be required in presses using separate dampening and inking systems, i.e., to account for variations in image density.
However, a major drawback of the use of emulsion inks is that emulsion inks are often unstable (i.e. the oil-based ink and water-based fountain solution separate into distinct liquid layers). Such instability is undesirable because it interferes with ink transfer. For example, if the emulsion ink is not stable enough, the oil-based ink and water-based fountain solution will separate prematurely, before reaching the printing plate, resulting in scumming and wash marks, as water released from the emulsion ink will interfere with ink transfer by flushing across image areas of the printing plate. However, if the emulsion is overly stable, it will (a) not release a sufficient amount of water to the printing plate to keep the non-image areas of the printing plate free of ink and/or (b) reduce the amount of emulsion ink fed to the printing plate. Accordingly, the emulsion ink is formulated to have a stability that is within a "window" between being too stable and too unstable for satisfactory lithographic printing. It has been found that suitable emulsion inks have a water content of at least 25% by weight.
Also, because the viscosity of lithographic inks is relatively high, about 10 to a few hundred poises, lithographic inks generally do not flow freely. As water is dispersed into a matrix of lithographic ink to produce emulsion inks, the flow properties further deteriorate, making the formation of a suitably stable emulsion ink difficult.